This invention relates to crude oil fouling. In one aspect the invention relates blending hydrocarbon streams to minimize fouling.
Fouling of process equipment is a continually costly problem in the petroleum and chemical industries. The fouling of heat exchangers by crude oils is the result of inorganic and organic carbonaceous deposits formation on the metal surface.
Deposition is caused by a combination of chemical reactions and physical changes that occur when crude oil is heated. These deposits increase pressure drop, block process flow, and cause the decrease of heat recovery from the process stream. Characterization of the deposits indicates the presence of inorganic material, infusable coke and asphaltenes.
All crude oils are composed of two major components, a low molecular weight oil fraction, and a high molecular weight fraction insoluble in paraffinic solvents. This fraction is called C.sub.7 -asphaltenes. As used herein the term "asphaltenes" refers to these paraffinic insoluble asphaltenes.
Fouling in crude oil heat exchangers in a function of crude oil composition, asphaltene presence, inorganic materials, process pressure and the temperature of the metal surface. Although there are a number of mechanisms which contribute to crude oil fouling, tests have shown asphaltene/oil incompatibility is a major contributing factor. Asphaltenes are characterized by a high average molecular weight and very broad molecular weight distribution (up to 5000).
The Thermal Fouling Tester (TFT) is widely used in the petroleum industry to measure crude oil fouling. The TFT test comprises circulating the crude oil through a miniaturized heat exchanger housing equipped with a carbon/steel heater tube while monitoring outlet temperatures of the crude oil. Fouling is determined by the decrease in fluid outlet temperature which is caused by deposit formation on the heater surface.
TFT unit does not simulate exactly refinery heat exchanger fouling. This test accelerates fouling by providing an increased inlet oil temperature in order to perform a test in the laboratory in a reasonable time (3-24 hours). However, the TFT is a valuable tool for research, investigating fouling mechanisms and for developing antifoulant.
Although antifoulant chemical may be employed to reduce or inhibit the fouling tendency, this type of treatment is expensive. Efforts have been made to blend low fouling crude with high fouling crude but, as will be discussed below, such efforts may in fact exacerbate the fouling tendency of the crude oil.